Electroless gold plating bath

ABSTRACT

The electroless gold plating bath includes a gold sulfate, a thiosulfate, ascorbic acid compounds, and hydrazine compounds, the hydrazine compounds being at least one selected from the group consisting of adipic dihydrazide, propionic hydrazide, hydrazine sulfate, hydrazine monohydrochloride, hydrazine dihydrochloride, hydrazine carbonate, hydrazine monohydrate, sebacic dihydrazide, dodecanediohydrazide, isophthalic dihydrazide, hydrazide, 3-hydro-2-naphtboic hydrazide benzophenone hydrazone, phenylhydrazine, benzylhydrazine monohydrochloride, methylhydrazine sulfate, and isopropylhydrazine hydrochloride.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No2021-168259 tiled on Oct. 13, 2021, the entire disclosure of which isincorporated by reference herein.

BACKGROUND

The present disclosure relates to an electroless gold plating bath.

Gold has high electrical conductivity next to silver and copper, and isexcellent in physical properties such as connectivity inthermocompression bonding, and is also excellent. in chemical propertiessuch as oxidation resistance and chemical resistance. Therefore, goldplating with gold is widely used as a final surface treatment method forcircuits on printed wiring boards, mounting portions and terminalportions of IC packages, and the like in the field of the electronicsindustry. In recent years, along with the improvement of electroniccomponents to be smaller and denser, electroless plating methodsattaining excellent functionality without the need of lead wiring havebeen used preferably.

One example of such an electroless plating method thus used is anElectroless Nickel Immersion Gold (ENIG) process. By using the ENIGprocess, a plating film sequentially including an electroless nickelplating film and a substitution gold plating film can be obtained.Moreover, for example, an Electroless Nickel Electroless PalladiumImmersion Gold (ENEPIG) process has been also used as such anelectroless plating method. By using the ENEPIG process, a plating filmsequentially including an electroless nickel plating film, anelectroless palladium plating film, and a substitution gold plating filmcan be obtained.

As a reduction substitution type electroless gold plating bath used inthese electroless plating methods, for example, an electroless goldplating bath has been proposed, which contains a water-soluble goldcompound, a complexing agent, and a reducing agent as well as polyvinylalcohol and/or polyvinylpyrrolidone added as a stabilizer. It isdescribed that such a configuration makes it possible to form a goodgold plating film only in metal portions and is preferably applicable togold plating for ceramics IC, packages, and the like (For example, seeJapanese Patent No. 2927142).

SUMMARY

In the known electroless gold plating bath described above, it isdifficult to deposit gold on the palladium plating film, so thatformation of the gold plating film would be insufficient in minuteportions such as terminal portions, thereby leading to difficulty informing a uniform gold plating film.

Moreover, in order to form a gold plating film with a sufficientthickness (thickness of 0.1 μm or thicker) on the nickel plating film,there has arisen the following problem: It is necessary to form theelectroless gold plating film with the electroless gold plating bathafter the formation of the substitution gold plating film on a platingobject, so that the gold plating needs to be performed in two steps.

In view of the problems described above, an object of the presentdisclosure is to provide an electroless gold plating bath, which iscapable of forming a uniform gold plating film with a sufficientthickness one step in both the ENIG process and the ENEPIG process.

In order to attain the object, the electroless gold plating bath of thepresent disclosure includes a gold sulfite, a thiosulfate, ascorbic acidcompounds, arid hydrazine compounds, the hydrazine compounds being atleast one selected from the group consisting of adipic dihydrazide,propionic hydrazide, hydrazine sulfate, hydrazine monohydrochloride,hydrazine dihydrochloride, hydrazine carbonate, hydrazine monohydrate,sebacic dihydrazide, dodecanediohydrazide, isophthalic dihydrazide,salicylic hydrazide 3-hydro-2-naphthoic hydrazide, benzophenonehydrazone, phenylhydrazine, benzylhydrazine monohydrochloride,methylhydrazine sulfate, isopropylhydrazine hydrochloride,1,1-dimethyhydrazine, 2-hydrazinobenzothiazole, acetohydrazide,2-hydroxyethylhydrazine, ethoxycarbonylhydrazine,methoxycarbonyldrazine, phenylhydrazine-4-sulfonic acid, andbenzohydrazide.

According to the present disclosure, it becomes possible to provide anelectroless gold plating bath, which is capable of forming a uniformgold plating film with a sufficient thickness in one step in both theENIG process and the ENEPIG process.

DETAILED DESCRIPTION

In the following, an electroless gold plating bath of the presentdisclosure will be described.

<Electroless Gold Plating Bath>

The electroless gold plating bath of the present disclosure is anelectroless gold plating bath including a gold sulfite as a gold source,a thiosulfate as a complexing agent, ascorbic acid compounds as areducing agent, and hydrazine compounds as a reducing agent.

(Gold Sulfite)

The electroless gold plating bath of the present disclosure is acyanide-free bath (non-cyanide bath), and includes, as the gold source,gold sulfite (e.g., sodium gold sulfite), which is a water-soluble goldcompound with no cyano group.

The concentration of the gold sulfite in the plating bath may bepreferably 0.5 g/L to 2 g/L based on gold. A concentration of the goldsulfite of less than 0.5 g/L would result in a low plating depositionrate, while a concentration of the gold sulfite of more than 2 g/L wouldresult in a poor adhesion of the gold plating film to the electrolessnickel plating film.

(Complexing Agent)

The complexing agent is used to stabilize the solubility of gold in theelectroless gold. plating bath. The electroless gold plating bath of thepresent disclosure includes a sulfur compound as the complexing agent.The sulfur compound may be a thiosulfate (such as sodium thiosulfate,potassium thiosulfate, ammonium thiosulfate, or the like) a sulfite(such as sodium sulfite, potassium sulfite, ammonium sulfite, or thelike). These complexing agents may be used solely, or a combination oftwo or more thereof may be used.

Moreover, the concentration of the thiosulfate (which may bethiosulfates) in the plating bath may be in a range of 0.5 g/L to 10g/L, preferably. If the concentration of the thiosulfate is less than0.5 g/L, the effect of the complex agent would be insufficient, and ifthe concentration of the thiosulfate is greater than 10 g/L, localcorrosion of the electroless nickel plating film could be increased,which would cause a gap bet peen the corroded portion of the electrolessnickel plating film and the electroless gold plating film, therebyreducing adhesion of the gold plating film to the electroless nickelplating film.

From the viewpoint of suppressing the decrease in adhesion of the goldplating film to the electroless nickel plating film and suppressing thecorrosion on the surfaces of the electroless nickel plating film andelectroless palladium plating film, it is preferable that a mass ratioof gold in the gold sulfite to thiosulfate be gold: thiosulfate=1; 0.5to 1; 10.

(Reducing Agent)

The reducing agent s used to reduce the gold sulfite, which is the goldsource, in order to precipitate gold. In the electroless gold platingbath of the present disclosure, the reducing agent includes hydrazinecompounds and ascorbic acid compounds in combination used at once.

The hydrazine compounds are for facilitating the formation of the goldplating film on the nickel plating film or the palladium plating film,especially for facilitating the formation of the gold plating film onthe palladium plating film in the ENEPIG process.

Examples of the hydrazine compounds include adipic dihydrazide,propionic hydrazide, hydrazine sulfate, hydrazine monohydrochloride,hydrazine dihydrochloride, hydrazine carbonate, hydrazine monohydrate,sebacic dihydrazide, dodecanediohydrazide, isophthalic dihydrazide,salicylic hydrazide, 3-hydro-2-naphthoic hydrazide, benzophenonehydrazone, phenylhydrazine, benzylhydrazine monohydrochloride,methylhydrazine sulfate, isopropylhydrazine hydrochloride,1,1-dimethyhydrazine, 2-hydrazinobenzothiazole, acetohydrazide,2-hydroxyethylhydrazine, ethoxycarbonylhydrazine,methoxycarbonylhydrazine, phenylhydrazine-4-sulfonic acid,benzohydrazide, and the like. These hydrazine compounds may be usedsolely, or a combination of two or more thereof may be used.

Moreover, the concentration of the hydrazine compound which may behydrazine compounds) in the plating bath may be in a range of 0.5 g/L,to 15 g/L, preferably. If the concentration of the hydrazine is lessthan 0.5 g/L, a plating rate would become insufficient. Moreover, ingeneral, the plating rate is increased proportionally to theconcentration of the reducing agent. However, if the concentration ofthe hydrazine is greater than 15 g/L, the plating rate would not beimproved so significantly regardless of the concentration, while bathstability of the plating bath would be deteriorated.

The ascorbic acid compounds are for improving the deposition property ofthe gold plating on the gold plating film deposited by the hydrazinecompounds so as to facilitate the formation of the gold parting film,and the ascorbic acid compounds make it possible to form a gold platingfilm with a sufficient thickness (of 0.1 μm or thicker).

Examples of the ascorbic acid compounds include alkali metal salts suchas ascorbic acid and sodium ascorbyl phosphate, alkali earth metal saltssuch as magnesium ascorbyl phosphate, esters such as ascorbic acid2-glucoside, and the like. These ascorbic acid compounds may be usedsolely, or a combination of two or more thereof may be used.

Moreover, the concentration of the ascorbic acid compound (which may beascorbic acid compounds) in the plating bath may be in a range of 1 g/Lto 20 g/L, preferably. If the concentration of the ascorbic acid is lessthan 1 g/L, the plating rate would become insufficient, and if theconcentration of the ascorbic acid is greater than 20 g/L, the platingrate would not be improved so significantly regardless of theconcentration, while bath stability of the plating bath would bedeteriorated.

The electroless gold plating bath of the present disclosure isconfigured as follows: The reducing agent includes a combination of thehydrazine compounds and the ascorbic acid compounds used at once, thehydrazine compounds being capable of facilitating the formation of thegold plating film on the nickel plating film and the palladium platingfilm, and the hydrazine compounds being capable of facilitating theformation of the gold plating film by improving the deposition propertyof the gold plating on the gold plating film deposited by the hydrazinecompounds, thereby making it possible to form a uniform gold platingfilm (that is, uniform in outer appearance and excellent in thedeposition property) with a sufficient thickness (of 0.1 μm or thicker)in one step process either in the ENIG process or ENEPIG process.

(Amine-Based Complexing Agent)

An amine-based complexing agent is for improving the adhesion of thegold plating film to the electroless nickel plating film, and theelectroless gold plating bath of the present disclosure includes anethyleneamine as the amine-based complexing agent.

Examples of the ethyleneamine include ethylenediamine,diethylenetriamine, triethylenetetramine, tetraethylenepentamine,pentaethylenhexamine, and the like. These polyethyleneamines may be usedsolely, or a combination of two or more thereof may be used,

Moreover, the concentration of the amine-based complexing agent in theplating bath may be in a range of 0.5 g/L to 10 g/L, preferably.

(Miscellaneous)

The plating bath of the present disclosure may be configured such thatthe electroless gold plating bath further includes a known additive(s)of various kinds, if necessary. Examples of the additive include ananti-corrosion agent, an electric conducting salt, and the like.

More specifically, the anti-corrosion agent is an agent for preventingthe corrosion on the surfaces of the nickel plating film and palladiumplating film. In the electroless gold plating bath of the presentdisclosure, the anti-corrosion agent is not particularly limited, butfor example, the anti-corrosion agent may be 1,2,3-benzotriazole,1,2,4-triazole, 3-amino-1,2,4-triazole, 3-mercapto-1,2,4-triazole,3-carboxamido-1,2,4-triazole, 3-aminopyrrolidine, 3-aminopyrazole, orthe like.

Moreover, the concentration of the anti-corrosion agent in the platingbath may be in a range of 0.1 g/L to 2 g/L, preferably.

Moreover, examples of the electric conducting salt include sodiumsuccinate, trisodium citrate, sodium malate, disodium malonate, sodiumoxalate, disodium glutarate, sodium tartrate, and the like.

Moreover, the concentration of the electric conducting salt in theplating bath may be in a range of 5 g/L to 100 g/L, preferably.

Moreover, the formation of the gold plating film with the sufficientthickness (of 0.1 μm or thicker) on the surface of the nickel platingfilm and/or the palladium plating film would result in deterioration ofthe adhesion of the gold plating film to the electroless nickel platingfilm and/or corrosion on the surface of the electroless nickel platingfilm and/or the electroless palladium plating film. Therefore, in orderto suppress the deterioration of the adhesion of the gold plating filmto the electroless nickel plating film and to suppress the corrosion onthe surface of the electroless nickel plating film and/or theelectroless palladium plating film, it is preferable that a mass ratiobetween the anti-corrosion agent and the amine-based complexing agent beanti-corrosion agent: amine-based complexing agent=1: 0.5 to 1: 10.

(pH)

It is preferable that pH of the electroless gold plating bath of thepresent disclosure is in a range of 6 to 9. If the pH is less than 6,the plating rate would be insufficient, and if pH is greater than 9, theplating bath would be unstable.

The pH of the plating bath can be adjusted by a pH adjuster such assodium hydroxide, potassium hydroxide, ammonia water, tetramethylammonium hydroxide, sulfuric acid, hydrochloric acid, boric acid,phosphoric acid, monocarboxylic acid, or dicarboxylic acid.

(Temperature of Plating Bath)

The temperature of the plating bath is not particularly limited, but maybe in a range of 50° C. to 80° C., preferably. If the temperature of theplating bath is less than 50° C., a deposition rate would be excessivelyslow, which would result in a long plating time, undesirably. If thetemperature of the plating bath is higher than 80° C., the depositionrate would be excessively fast, which would produce coarse films, whichwould cause warping of a substrate due to thermal shrinkage of the filmsafter the plating. Thus, such a high temperature of the plating bath isnot preferable.

(Plating Object)

The electroless gold plating bath of the present disclosure isapplicable to any kinds of plating objects, and is applicable to platingobjects which have been treated with known electroless gold platingprocesses (for example, wiring circuits, terminal portions, and the likeof electronic parts such as printed wiring boards, ceramics substrates,semiconductor substrates, IC packages, and the like).

<Electroless Gold Plating Process>

The electroless gold plating bath according to the present disclosureis, for example, electroless gold plating process in the ENIG processand the ENEPIG process. For example, an electroless gold plating filmcan be farmed on a palladium plating film by implementing theelectroless gold plating bath according to the present disclosure insuch a way that a surface of a palladium plating film is subjected tothe electroless gold plating process by immersing the palladium platingfilm in the electroless gold plating film. The temperature of theelectroless gold plating process is adjusted to the bath temperature ofthe electroless gold plating bath described above.

Moreover, a process time of the electroless gold plating process is notparticularly limited and may be adjusted as appropriate to attain adesired film thickness. More specifically, the electroless gold platingprocess may be, for example in a range of 30 sec to 15 hours,approximately.

EXAMPLES

The following describes the present disclosure more specifically basedon Examples and Comparative Examples. However, the present invention isnot limited to the following Examples.

First, to confirm that the electroless gold plating bath according tothe present disclosure works usefully, the deposition property of goldplating films formed by plating process using the electroless goldplating bath according to the present disclosure was evaluated.

<Evaluation of Deposition Property of Gold Plating on ElectrolessNickel-Phosphorus Plating Film (with a Phosphorus Concentration of 6 to8% in the Film)>

(Preparation of Plating Bath)

An electroless gold plating bath of Example 1 was prepared by mixing andstirring gold sulfite (sodium gold sulfite), sodium succinate as theelectric conducting salt, sodium thiosulfate and disodium sulfite as thecomplexing agent, 1,2,3-benzotriazole as the anti-corrosion agent,ethylenediamine as the amine-based complexing agent, and ascorbic addand adipic dihydrazide as the reducing agent with concentrations listedin Table 1. The plating bath was set to a temperature (i.e., temperatureof the plating processing) of 70° C., and the pH was set to 7.5.

TABLE 1 Composition of Plating Bath Example 1 Gold Sulfite Sodium GoldSulfite g/L 1 (as Au) Electric Conducting Salt Sodium Succinate g/L 30Complexing Agent Sodium Thiosulfate g/L 1 Disodium Sulfite g/L 30Anti-Corrosion Agent 1,2,3-Benzotriazole g/L 1 Amine-based ComplexingEthylenediamine g/L 1 Agent Reducing Agent (Ascorbic Ascorbic Acid g/L 5Acid Compounds) Reducing Agent (Hydrazine Adipic Dihydrazide g/L 5Compounds) Process Temperature (° C.) 70 pH 7.5

(Pretreatment)

A TEG wafer with electrodes made from an aluminum-copper alloy, which isan aluminum alloy, was provided and the electrodes were treated with thefollowing pretreatment steps 1 to 5 in this order.

Step 1: A substrate (TEG wafer) was treated with a degreasing cleaningtreatment (50° C., 300 sec) with a degreasing cleaner (EPITHAS MCL-16(product name) available from C. Uyemura & Co., Ltd.).

Step 2: Next, the substrate was treated with a pickling treatment (21°C., 30 sec) with a nitric acid solution of 30 wt %, thereby forming anoxide film on the surface of the substrate.

Step 3: Next, the substrate was treated with a primary zincate treatment(21° C., 20 sec) with a zincate treatment bath (with EPITHAS MCT-51(product name) available from C. Uyemura & Co., Ltd.).

Step 4: The substrate was treated with a pickling treatment (21° C., 60sec) with the nitric acid solution of 30 wt %, so as to peel off thezincate film and forming an oxide film on the surface of the substrate.

Step 5: Next, the substrate was treated with a secondary zincatetreatment (21° C. 40 sec) with a zincate treatment bath (with EPITHASMCT-51 (product name) available from C. Uyemura & Co., Ltd.).

(Plating Processing)

Next, the substrate thus prepared was treated with an electrolessplating process (80° C., 15 min) with a nickel-phosphorus pitting bath(with EPITHAS NPR-18 (product name) available from C. Uyemura & Co.Ltd.), thereby forming a nickel plating film (a nickel plating film witha phosphorus concentration of 6% to 8% in the film and a thickness of 3μm) on the substrate.

Next, the substrate with the nickel plating film thereon was immersedfor 15 min in the plating bath prepared as above, thereby forming anelectroless gold plating film on the plating object,

(Plating Deposition Property)

The substrate thus treated with the electroless gold plating process wasvisually observed in terms of tonal change of the outer appearance ofthe substrate due to the gold deposition, so as to evaluate thedeposition property of the gold plating film formed by the platingprocess. The evaluation found that the surface color of the substratehad been changed to yellow, confirming gold deposition.

<Evaluation of Deposition Property of Gold Plating on ElectrolessNickel-Phosphorus Plating Film (with a Phosphorus Concentration of 2 to4% in the film)>

A TEG wafer with electrodes made from an aluminum-copper alloy, which isan aluminum alloy, was provided and the electrodes were treated with theaforementioned pretreatment steps 1 to 5 in this order.

Next, the substrate thus prepared was treated with an electrolessplating process (80° C., 15 min with a nickel-phosphorus plating bath(with EPITHAS NLL-1 (product name available from C. Uyemura & Co.,Ltd.), thereby forming a nickel plating film (a nickel plating film witha phosphorus concentration of 2% to 4% in the film and a thickness of 3μm) on the substrate.

Next, the substrate with the nickel plating film thereon was immersedfor 15 min in the plating bath prepared as above, thereby forming anelectroless gold plating, film on the plating object.

The substrate was evaluated in terms of the deposition property of theplating, finding a surface color change of the substrate to yellow andthereby confirming gold deposition.

<Evaluation of Deposition Property of Gold Plating on ElectrolessNickel-Bismuth Alloy Plating Film>

A TEG wafer with electrodes made from an aluminum-copper alloy, which isan aluminum alloy, was provided and the electrodes were treated with theaforementioned pretreatment steps 1 to 5 in this order.

Next, the substrate thus prepared was treated with an electrolessplating process (80° C., 15 min) with a nickel-bismuth plating bath(EPITHAS KSB-18 (product name) available from C. Uyemura & Co., Ltd.),thereby forming a nickel plating film (with thickness of 3 μm on thesubstrate.

Next, the substrate with the nickel plating film thereon was immersedfor 15 win in the plating bath prepared as above, thereby forming anelectroless gold plating film on the plating object.

The substrate was evaluated in terms of the deposition property of theplating, finding a surface color change of the substrate to yellow andthereby confirming gold deposition.

<Evaluation of Deposition Property Gold Plating on Electroless PalladiumPlating Film>

A TEG wafer with electrodes made from an aluminum-copper alloy, which isan aluminum alloy, was provided and the electrodes were treated with theaforementioned pretreatment steps 1 to 5 in this order.

Next, the substrate thus prepared was treated with an electrolessplating process (80° C., 15 win) with a nickel-phosphorus plating bath(with EPITHAS NPR-18 (product name) available from C. Uyemura & Co.,Ltd), thereby forming a nickel plating film (with a thickness of 3 μm)on the substrate.

Next, the substrate was treated with an electroless plating process (56°C., 15 min) with a palladium plating bath (with EPITHAS TFP-25 (productname) available from C. Uyemura & Co., Ltd.), thereby forming apalladium plating film (with a thickness of 0.3 μm) on the nickelplating film.

Next, the substrate with the palladium plating film was immersed for 15min in the plating bath prepared as above, thereby forming anelectroless gold plating film on the plating object.

The plating object was evaluated in terms of the deposition property ofthe plating, finding a surface color change of the substrate to yellowand thereby confirming gold deposition.

<Evaluation of Deposition Property of Gold Plating on ElectrolessPalladium-Phosphorus Plating Film>

A TEG wafer with electrodes made from an aluminum-copper alloy, which isan aluminum alloy, was provided and the electrodes were treated with theaforementioned pretreatment steps 1 to 5 in this order.

Next, the substrate thus prepared was treated with an electrolessplating process (80° C., 15 min) with a nickel-phosphorus plating bath(with EPITHAS NPR-18 (product name available from C. Uyemura & Co.,Ltd.), thereby forming a nickel plating film (with a thickness of 3 μm)on the substrate.

Next, the substrate thus prepared was treated with an electrolessplating process (50° C., 15 min) with a palladium-phosphorus platingbath (with EPITHAS TFP-30 (product name) available from C. Uyemura &Co., Ltd.), thereby forming a palladium plating film (with a thicknessof 0.15 μm) on the nickel plating film.

Next, the substrate with the palladium plating film was immersed for 15min in the plating bath prepared as above, thereby forming areelectroless gold plating film on the plating object.

The substrate was evaluated in terms of the deposition property of theplating, finding a surface color change of the substrate to yellow andthereby confirming gold deposition.

=Evaluation of Deposition Property of Gold Plating on ElectrolessPlatinum Plating Film>

A TEG wafer with electrodes made from an aluminum-copper alloy, which isan aluminum alloy, was provided and the electrodes were treated with theaforementioned pretreatment steps 1 to 5 in this order.

Next, the substrate thus prepared was treated with an electrolessplating process (80° C., 15 min) with a nickel-phosphorus plating bath(with EPITHAS NPR-18 (product name) available from C. Uyemura & Co.,Ltd.), thereby forming a nickel plating film (with a thickness of 3 μm)on the substrate.

Next, the substrate thus prepared was treated with an electrolessplating process (78° C., 7 min) with a platinum plating bath (withEPITHAS TAE-30 (product name) available from C. Uyemura & Co., Ltd.),thereby forming a platinum plating film (with a thickness of 0.2 μm) onthe nickel plating film.

Next, the substrate with the platinum plating film thereon was immersedfor 15 min in the plating bath prepared as above, thereby forming anelectroless gold plating film on the plating object.

The substrate was evaluated in terms of the deposition property of theplating, finding a surface color change of the substrate to yellow andthereby confirming gold deposition.

<Evaluation of Deposition Property of Gold Plating on ElectrolessCobalt-Tungsten-Phosphorus Plating Film>

A TEG wafer with electrodes made from an aluminum-copper alloy, which isaluminum alloy, was provided and the electrodes were treated with theaforementioned pretreatment steps 1 to 5 in this order.

Next, the substrate thus prepared was treated with an electrolessplating process (75° C., 60 min) with a cobalt-tungsten-phosphorusplating bath (with EPITHAS HWP-5 (product name) available from C.Uyemura &. Co., Ltd.), thereby thrilling a cobalt alloy plating film(with a thickness of 0.3 μm) on the substrate.

Next, the substrate with the cobalt alloy plating film thereon wasimmersed for 15 min in the plating bath prepared as above, therebyforming an electroless gold plating film on the plating object.

The substrate was evaluated in terms of the deposition property of theplating, finding a surface color change of the substrate to yellow andthereby confirming gold deposition.

<Evaluation of Deposition Property of Gold Plating on ElectrolessCobalt-Tungsten-Boron Plating Film>

A TEG wafer with electrodes made from an aluminum-copper alloy, which isan aluminum alloy, was provided and the electrodes were treated with theaforementioned pretreatment steps 1 to 5 in this order.

Next, the substrate thus prepared was treated with an electrolessplating process (75° C., 60 min) with a cobalt-tungsten-boron platingbath (with EPITHAS HWB-31 (product name) available from C. Uyemura &Co., Ltd.), thereby thrilling a cobalt alloy plating him (with athickness of 0.5 μm) on the substrate.

Next, the substrate with the cobalt alloy plating film thereon wasimmersed for 15 min in the plating bath prepared as above, therebyforming an electroless gold plating film on the plating object.

The substrate was evaluated in terms of the deposition property of theplating, finding a surface color change of the substrate to yellow andthereby confirming gold deposition.

As described above, it was confirmed that gold was successfullydeposited by the use of the electroless gold plating bath according tothe present disclosure in any of the cases where the base plating filmwas the electroless nickel-phosphorus plating film, the electrolessnickel-bismuth plating film, the electroless palladium plating film, theelectroless palladium-phosphorus plating film, the electroless platinumplating film, or the electroless cobalt alloy plating film, therebyconfirming that the electroless gold plating bath according to thepresent disclosure could work usefully.

<Formation of Electroless Gold Plating Film in the ENIG Process andENEPIG Process>

Next, properties of gold plating films formed by the plating processwith the electroless gold plating bath in the ENIG Process and ENEPIGprocess were evaluated.

Examples 1 to 48 and Comparative Examples 1 to 9

(Preparation of Plating Bath)

Electroless gold plating baths of Examples 1 to 48 and ComparativeExamples 1 to 9 were prepared by mixing and stirring a gold sulfite(sodium gold sulfite), sodium succinate as the electric conducting salt,sodium thiosulfate and disodium sulfite as the complexing agent,1,2,3-benzotriazole as the anti-corrosion agent, an amine-basedcomplexing agent, and ascorbic acid compounds and hydrazine compounds asthe reducing agent with concentrations listed in Tables 2 to 6. Theplating bath was set to a temperature (i.e., temperature of the platingprocessing) in a range of 50° C. to 80° C., and the pH was set in arange of 6 to 9.

(Formation of Electroless Gold Plating Film in the ENIG Process)

A TEG wafer with electrodes made from an aluminum-copper alloy, which isan aluminum alloy, was provided and the electrodes were treated with theaforementioned pretreatment steps 1 to 5 in this order.

Next, the substrate thus prepared was treated with an electrolessplating process (80° C., 15 min) with a nickel-phosphorus plating bath(with EPITHAS NPR-18 (product name) available from C. Uyemura & Co.,Ltd.), thereby forming a nickel plating blur (with a thickness of 3 μm)on the substrate.

Next, the substrate with the nickel plating film thereon was immersedfor 15 min in the plating bath of one of Examples 1 to 48 andComparative Examples 1 to 9, thereby forming an electroless gold platingfilm on the nickel plating film.

(Formation of Electroless Gold Plating Film in the ENEPIG process)

A TEG wafer with electrodes made from an aluminum-copper alloy, which isan aluminum alloy, was provided and the electrodes were treated with theaforementioned pretreatment steps 1 to 5 in this order.

Next, the substrate thus prepared was treated with an electrolessplating process (80° C., 15 min) with a nickel-phosphorus plating bath(with EPITHAS NPR-18 (product name) available from C. Uyemura & Co.,Ltd.), thereby forming a nickel plating film (with a thickness of 3 μm)on the substrate.

Next, the substrate was treated with an electroless plating process (56°C., 15 min) with a palladium plating bath (with EPITHAS TFP-25 (productname) available from C. Uyemura & Co., Ltd.), thereby forming apalladium plating film (with a thickness of 0.3 μm) on the nickelplating film.

Next, the substrate with the nickel plating film thereon was immersedfor 15 min in the plating bath of one of Examples 1 to 48 andComparative Examples 1 to 9, thereby forming an electroless gold platingfilm on the palladium plating film.

(Outer Appearance Unevenness)

The surfaces of the electroless gold plating film were visually observedto evaluate in such a way that films with uniform gold color outerappearance were evaluated as good, and flints with color changes to redinstead of gold were evaluated as poor. The results of the evaluationare presented in Tables 2 to 6.

(Film Peeling-Off of the Electroless Gold Plating Film)

A sample of the electroless gold plating film with a size of 2.5 cm×2.5cm was prepared. An adhesive cellophane tape with width of approximately2 cm was adhered to a surface of the electroless gold plating, film andmanually peeled off from the surface. If the electroless gold platingfilm was not peeled off together with the tape, the electroless goldplating film was evaluated as good, and if peeled off, the electrolessgold plating film was evaluated as poor. The results of the evaluationare presented in Tables 2 to 6.

(Deposition Property)

The surfaces of the electroless gold plating film were observed with ascanning electron microscope (SEM, available from JEOL Ltd.) to evaluatein such a way that films with uniform gold plating deposition wereevaluated as good, and films with non-uniform gold plating deposition,in which gold deposition was partially unsuccessful, were evaluated aspoor. The results of the evaluation are presented in Tables 2 to 6.

(Corrosivity)

By using COPKIA RIP AU-1 (available from C. Uyemura & Co., Ltd.), theelectroless gold plating film was peeled off, and a base film of theelectroless gold plating film (the surface of the electroless nickelplating film in the case of the ENIG process and the electroless nickelplating film under the electroless palladium plating film in the case ofENEPIG process) was observed by a scanning electron microscope, and across-section thereof was observed with a focused ion beam (FIB) deviceavailable from Hitachi High-Tech Corporation), evaluating as poor if theelectroless nickel plating film has corrosion on the surface or thecross-section thereof (that is, if the electroless nickel plating filmhas a hole on the surface or a hole or staining on the cross-section)and evaluating as good if the electroless nickel plating film had nosuch corrosion. The results of the valuation are presented in Tables 2to 6.

(Measurement or Thickness Electroless Gold Plating Film)

Next, the thicknesses of the electroless gold plating films weremeasured by using an X-ray fluorescence thickness meter available fromFISCHER INSTRUMENTS K.K. The results of the evaluation are presented inTables 2 to 6.

TABLE 2 Compositions of Plating Baths Examples 1 2 3 4 5 6 7 8 9 10 1112 Gold Sulfite Sodium Gold Sulfite (as Au) g/L 1 1 1 1 1 1 1 1 1 1 1 1Electric Conducting Salt Sodium Succinate g/L 30 30 30 30 30 30 30 30 3030 30 30 Complexing Agent Sodium Thiosulfate g/L 1 1 1 1 1 1 1 1 1 1 1 1Disodium Sulfite g/L 30 30 30 30 30 30 30 30 30 30 30 30 Anti-CorrosionAgent 1,2,3-Benzotriazole g/L 1 1 1 1 1 1 1 1 1 1 1 1 Amine-basedComplexing Ethylenediamine g/L 1 1 1 1 1 1 1 1 1 1 1 1 Reducing AgentAscorbic Acid g/L 5 5 5 20 1 5 5 5 5 (Ascorbic Acid Ascorbic Acid2-Glucoside g/L 5 Compounds) Magnesium Ascorbyl g/L 5 Phosphate SodiumAscorbyl Phosphate g/L 5 Reducing Agent Adipic Dihydrazide g/L 5 15 0.55 5 5 5 5 (Hydrazine Compounds) Propionic Hydrazide g/L 5 HydrazineSulfate g/L 5 Hydrazine g/L 5 Monohydrochloride HydrazineDihydrochloride g/L 5 Hydrazine Carbonate g/L Gold: Sodium Thiosulfate1:1 Anti-Corrosion Agent: 1:1 Amine-based Complexing Agent ProcessTemperature (° C.) 70 pH 7.5 ENIG Process Uniform Outer Appearance GoodGood Good Good Good Good Good Good Good Good Good Good No Peeling-Off ofElectroless Good Good Good Good Good Good Good Good Good Good Good GoodGold Plating Film Deposition Property Good Good Good Good Good Good GoodGood Good Good Good Good No Corrosion Good Good Good Good Good Good GoodGood Good Good Good Good Film Thickness of Electroless 0.1 0.1 0.1 0.10.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Gold Plating Film (μm) ENEPIG ProcessUniform Outer Appearance Gone Good Good Good Good Good Good Good GoodGood Good Good No Peeling-Off of Electroless Good Good Good Good GoodGood Good Good Good Good Good Good Gold Plating Film Deposition ProperlyGood Good Good Good Good Good Good Good Good Good Good Good No CorrosionGood Good Good Good Good Good Good Good Good Good Good Good FilmThickness of Electroless 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1Gold Plating Film (μm)

TABLE 3 Compositions of Plating Baths Examples 13 14 15 16 17 18 19 2021 22 23 24 Gold Sulfite Sodium Gold Sulfite g/L 1 1 1 1 1 1 1 1 1 1 1 1(as Au) Electric Conducting Salt Sodium Succinate g/L 30 30 30 30 30 3030 30 30 30 30 30 Complexing Agent Sodium Thiosulfate 1 1 1 1 1 1 1 1 11 1 1 Disodium Sulfite g/L 30 30 30 30 30 30 30 30 30 30 30 30Anti-Corrosion Agent 1,2,3-Benzotriazole g/L 1 1 1 1 1 1 1 1 1 1 1 1Amine-based Complexing Agent Ethylenediamine g/L 1 1 1 1 1 1 1 1 1 1 1 1Reducing Agent (Ascorbic Ascorbic Acid g/L 5 5 5 5 5 5 5 5 5 5 5 5 AcidCompounds) Reducing Agent Hydrazine Carbonate g/L 5 (HydrazineCompounds) Hydrazine Monohydrate g/L 5 Sebacic Dihydrazide g/L 5Dodecanediohydrazide g/L 5 Isophthalic Dihydrazide g/L 5 SalicylicHydrazide g/L 5 3-Hydro-2- g/L 5 Naphthoic Hydrazide, Benzophenone g/L 5Hydrazone Phenylhydrazine g/L 5 Benzylhydrazine g/L 5 MonohydrochlorideMethylhydrazine Sulfate g/L 5 Isopropylhydrazine g/L 5 HydrochlorideGold: Sodium Thiosulfate 1:1 Anti-Corrosion Agent: 1:1 Amine-basedComplexing Agent Process Temperature (° C.) 70 pH 7.5 ENIG ProcessUniform Outer Appearance Good Good Good Good Good Good Good Good GoodGood Good Good No Peeling-Off of Electroless Good Good Good Good GoodGood Good Good Good Good Good Good Gold Plating Film Deposition PropertyGood Good Good Good Good Good Good Good Good Good Good Good No CorrosionGood Good Good Good Good Good Good Good Good Good Good Good FilmThickness of Electroless 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1Gold Plating Film (μm) ENEPIG Process Uniform Outer Appearance Good GoodGood Good Good Good Good Good Good Good Good Good No Peeling-Off ofElectroless Good Good Good Good Good Good Good Good Good Good Good GoodGold Plating Film Deposition Property Gaud Good Good Good Good Good GoodGood Good Good Good Good No Corrosion Good Good Good Good Good Good GoodGood Good Good Good Good Film Thickness of Electroless 0.1 0.1 0.1 0.10.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Gold Plating Film (μm)

TABLE 4 Compositions of Plating Baths Examples 25 26 27 28 29 30 31 3233 34 35 36 Gold Sulfite Sodium Gold Sulfite (as Au) g/L 1 1 1 1 1 1 1 11 1 0.5 2 Electric Conducting Salt Sodium Succinate g/L 30 30 30 30 3030 30 30 30 30 30 30 Complexing Agent Sodium Thiosulfate g/L 1 1 1 1 1 11 1 0.5 10 1 1 Disodium Sulfite g/L 30 30 30 30 30 30 30 30 30 30 30 30Anti-Corrosion Agent 1,2,3-Benzotriazole g/L 1 1 1 1 1 1 1 1 1 1 1 1Amine-based Complexing Ethylenediamine g/L 1 1 1 1 1 1 1 1 1 1 1 1Reducing Agent (Ascorbic Ascorbic Acid g/L 5 5 5 5 5 5 5 5 5 5 5 5 AcidCompounds) Reducing Agent Adipic Dihydrazide g/L 5 5 5 5 (HydrazineCompounds) 1,1-Dimethylhydrazine g/L 5 2-Hydrazinobenzothiazole g/L 5Acetohydrazide g/L 5 2-Hydroxyethylhydrazine g/L 5Ethoxycarbonylhydrazine g/L 5 Methoxycarbonylhydrazine g/L 5Phenylhydrazine-4- g/L 5 Sulfonic acid Benzohydrazine g/L 5 Gold: SodiumThiosulfate 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 2:1 1:10 1:2 2:1Anti-Corrosion Agent: Amine-based Complexing Agent 1:1 ProcessTemperature (° C.) 70 pH 7.5 ENIG Process Uniform Outer Appearance GoodGood Good Good Good Good Good Good Good Good Good Good No Peeling-Off ofElectroless Good Good Good Good Good Good Good Good Good Good Good GoodGold Plating Film Deposition Property Good Good Good Good Good Good GoodGood Good Good Good Good No Corrosion Good Good Good Good Good Good GoodGood Good Good Good Good Film Thickness of Electroless 0.1 0.1 0.1 0.10.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Gold Plating Film (μm) ENEPIG ProcessUniform Outer Appearance Good Good Good Good Good Good Good Good GoodGood Good Good No Peeling-Off of Electroless Good Good Good Good GoodGood Good Good Good Good Good Good Gold Plating Film Deposition PropertyGood Good Good Good Good Good Good Good Good Good Good Good No CorrosionGood Good Good Good Good Good Good Good Good Good Good Good FilmThickness of Electroless 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1Gold Plating Film (μm)

TABLE 5 Compositions of Plating Baths Examples 37 38 39 40 41 42 43 4445 46 47 48 Gold Sulfite Sodium Gold Sulfite g/L 1 1 1 1 1 1 1 1 1 1 1 1(as Au) Electric Conducting Salt Sodium Succinate g/L 30 30 30 30 30 3030 30 30 30 30 30 Complexing Agent Sodium Thiosulfate g/L 1 1 1 1 1 1 11 1 1 1 1 Disodium Sulfite g/L 30 30 30 30 30 30 30 30 30 30 30 30Anti-Corrosion Agent 1,2,3-Benzotriazole g/L 1 1 1 1 1 1 0.1 2 1 1 1 1Amine-based Complexing Ethylenediamine g/L 0.5 10 1 1 1 1 1 1 AgentDiethylenetriamine g/L 1 Triethylenetetramine g/L 1Tetraethylenepentamine g/L 1 Pentaethylenehexamine g/L 1 Reducing Agent(Ascorbic Ascorbic Acid g/L 5 5 5 5 5 5 5 5 5 5 5 5 Acid Compounds)Reducing Agent Adipic Dihydrazide g/L 5 5 5 5 5 5 5 5 5 5 5 5 (HydrazineCompounds) Gold: Sodium Thiosulfate 1:1 Anti-Corrosion Agent: 1:1 1:11:1 1:1 2:1 1:10 1:10 2:1 1:1 1:1 1:1 1:1 Amine-based Complexing AgentProcess Temperature (° C.) 70 70 70 70 70 70 70 70 70 70 50 80 pH 7.57.5 7.5 7.5 7.5 7.5 7.5 7.5 6 9 7.5 7.5 ENIG Process Uniform OuterAppearance Good Good Good Good Good Good Good Good Good Good Good GoodNo Peeling-Off of Electroless Good Good Good Good Good Good Good GoodGood Good Good Good Gold Plating Deposition Property Good Good Good GoodGood Good Good Good Good Good Good Good No Corrosion Good Good Good GoodGood Good Good Good Good Good Good Good Film Thickness of Electroless0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Gold Plating Film (μm)ENEPIG Process Uniform Outer Appearance Good Good Good Good Good GoodGood Good Good Good Good Good No Peeling-Off of Electroless Good GoodGood Good Good Good Good Good Good Good Good Good Gold PlatingDeposition Property Good Good Good Good Good Good Good Good Good GoodGood Good No Corrosion Good Good Good Good Good Good Good Good Good GoodGood Good Film Thickness of Electroless 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.10.1 0.1 0.1 0.1 Gold Plating Film (μm)

TABLE 6 Compositions of Plating Baths Comparative Examples 1 2 3 4 5 6 78 9 Gold Sulfite Sodium Gold Sulfite (as Au) g/L 1 1 1 1 5 1 1 1 1Electric Conducting Salt Sodium Succinate g/L 30 30 30 30 30 30 30 30 30Complexing Agent Sodium Thiosulfate g/L 1 1 15 1 1 1 1 1 DisodiumSulfite g/L 30 30 30 30 30 30 30 30 30 Anti-Corrosion Agent1,2,3-Benzotriazole g/L 1 1 1 1 1 1 1 3 Amine-based Complexing AgentEthylenediamine g/L 1 1 1 1 1 15 1 1 Reducing Agent (Ascorbic AscorbicAcid g/L 5 5 5 5 5 5 5 5 Acid Compounds) Reducing Agent AdipicDihydrazide g/L 5 5 5 5 5 5 5 5 (Hydrazine Compounds) Gold: SodiumThiosulfate 1:1 1:1 — 1:15 5:1 1:1 1:1  1:1 1:1 Anti-Corrosion Agent:Amine-based Complexing Agent 1:1 1:1 1:1 1:1  1:1 — 1:15 — 3:1 ProcessTemperature (° C.) 70 70 70 70 70 70 70 70 70 pH 7.5 7.5 7.5 7.5 7.5 7.57.5 7.5 7.5 ENIG Process Uniform Outer Appearance Good Good Good GoodGood Good Good Good Good No Peeling-Off of Electroless Good Good GoodPoor Poor Poor Poor Poor Poor Gold Plating Film Deposition Property PoorPoor Poor Good Good Good Good Good Good No Corrosion Good Good Good PoorGood Good Poor Poor Good Film Thickness of Electroless 0.1 0.03 0.03 0.10.1 0.1 0.1 0.1 0.1 Gold Plating Film (μm) ENEPIG Process Uniform OuterAppearance Poor Good Poor Good Good Good Good Good Good No Peeling-Offof Electroless Good Good Good Good Good Good Good Good Good Gold PlatingFilm Deposition Property Poor Poor Poor Good Good Good Good Good Good NoCorrosion Good Good Good Poor Good Good Poor Poor Good Film Thickness ofElectroless 0.1 0.03 0.03 0.1 0.1 0.1 0.1 0.1 0.1 Gold Plating Film (μm)

As demonstrated in Tables 2 to 5, the electroless gold plating baths ofExamples 1 to 48, including a gold sulfite, a thiosulfate, ascorbic acidcompounds, and hydrazine compounds, wherein a combination of thehydrazine compounds and the ascorbic acid serve as a reducing agent,make it possible to form a uniform gold plating film (that is, uniformin outer appearance and excellent in the deposition property) with asufficient thickness (of 0.1 μm or thicker) by a single-step processeither in the ENIG process or ENEPIG process, because the hydrazinecompounds capable of facilitating the formation of the gold plating filmon the nickel plating film or the palladium plating film and theascorbic acid compounds capable of facilitating the formation of thegold plating film by improving the deposition property of the goldplating on the gold plating film deposited by the hydrazine are used atonce.

On the other hand, the Comparative Example 1 without hydrazine compoundswas poor in the outer appearance and deposition property, and the goldplating film thereof was not uniform, its understood from Table 6.

Moreover, the Comparative Example 2 without ascorbic acid compounds waspoor in the deposition property and the gold plating film thereof wasnot uniform.

Moreover, the Comparative Example 3 without a sodium thiosulfate wasunstable in gold solubility in the electroless gold plating bath andpoor in the outer appearance and deposition property, and the goldplating film thereof was not uniform.

Moreover, the Comparative Example 4, in which the mass ratio of the goldin the gold sulfite to the sodium thiosulfate was 1: 15 out of the rangeof gold: sodium thiosulfate=1: 0.5 to 1: 10, was poor in the adhesion ofthe gold plating film to the electroless nickel plating film andcorrosion occurred on the surface of the electroless nickel platingfilm.

Moreover, the Comparative Example 5, in which the mass ratio of the goldin the gold sulfite to the sodium thiosulfate was 5: 1 out of the rangeof gold: sodium thiosulfate=1: 0.5 to 1: 10, was poor in the adhesion ofthe gold plating film to the electroless nickel plating film.

Moreover, the Comparative Example 6 without ethylene diamine was poor inthe adhesion of the gold plating film to the electroless nickel platingfilm.

Moreover, the Comparative Example 7, in which the mass ratio of theanti-corrosion agent to amine-based complexing agent was 1: 15 out ofthe range of anti-corrosion agent: amine-based complexing agent=1: 0.5to 1: 10. was poor in the adhesion of the gold plating film to theelectroless nickel plating film and corrosion occurred on the surface ofthe electroless nickel plating film.

Moreover, the Comparative Example 8 without an anti-corrosion agent waspoor in the adhesion of the gold plating film to the electroless nickelplating film, because local corrosions increased on the electrolessnickel plating film, causing gaps between the corroded portion of theelectroless nickel plating film and the electroless gold plating film.

Moreover, the Comparative Example 9, in which the mass ratio of theanti-corrosion agent to the amine-based complexing agent was 3: 1 out ofthe range of anti-corrosion agent: amine-based complexing agent=1: 0.5to 1: 10, was poor in the adhesion of the gold plating film to theelectroless nickel plating film.

INDUSTRIAL APPLICABILITY

The electroless gold plating bath according to the present disclosure issuitably applicable especially to formation of an electroless goldplating film in the ENIG process and the ENEPIG process.

What is claimed is:
 1. An electroless gold plating bath, comprising agold sulfite, a thiosulfate, ascorbic acid compounds, and hydrazinecompounds, the hydrazine compounds being at least one selected from thegroup consisting of adipic dihydrazide, propionic hydrazide, hydrazinesulfate, hydrazine monohydrochloride, hydrazine dihydrochloride,hydrazine carbonate, hydrazine monohydrate, sebacic dihydrazide,dodecanediohydrazide, isophthalic dihydrazide, salicylic hydrazide,3-hydro-2-naphthoic hydrazide, benzophenone hydrazone, phenylhydrazine,benzylhydrazine monohydrochloride, methylhydrazine sulfate,isopropylhydrazine hydrochloride, 1,1-dimethyhydrazine,2-hydrazinobenzothiazole, acetohydrazide, 2-hydroxyethylhydrazine,ethoxycarbonylhydrazine, methoxycarbonylhydrazine,phenylhydrazine-4-sulfonic acid, and benzohydrazide.
 2. The electrolessgold plating bath of claim 1, wherein the ascorbic acid compounds are atleast one selected from the group consisting of ascorbic acid, sodiumascorbyl phosphate, magnesium ascorbyl phosphate, and ascorbic acid2-glucoside.
 3. The electroless gold plating bath of claim 1, wherein aconcentration of the hydrazine compounds is in a range of 0.5 g/L to 15g/L and a concentration of the ascorbic acid compounds is in a range of1 g/L to 20 gL.
 4. The electroless gold plating bath of claim 1, whereina mass ratio of gold in the gold sulfite to the thiosulfate is in arange of gold: thiosulfate=1: 0.5 to 1:
 10. 5. The electroless goldplating bath of claim 1, further comprising: an anti-corrosion agent andan amine-based complexing agent, wherein a mass ratio of theanti-corrosion agent to the amine-based complexing agent is in a rangeof anti-corrosion agent: amine-based complexing agent=1: 0.5 to 1: 10.